The present invention relates to semiconductor devices, and more particularly, to image sensors and transistors.
Image sensors are semiconductor devices that may convert an optical image into an electric signal. With recent developments of computer and communication industries, image sensors have become widely used in various fields, including, for example, digital cameras, camcorders, personal computer systems (PCSs), game devices, surveillance cameras, medical micro cameras, and robots, among others.
Image sensors can be classified into a number of types, including, for example, charge coupled device (CCD) image sensors and CMOS image sensors. CMOS image sensors may be easy to drive and may be fabricated using a conventional CMOS fabrication technology. In this regard, a manufacturing cost of CMOS image sensors may be low. CMOS image sensors may be implemented to include various structures. For example, some CMOS image sensors may be fabricated using four transistors, while some CMOS image sensors may be fabricated using three transistors. A transistor structure for pixels of CMOS image sensors may be fabricated using a CMOS fabrication process.
In a conventional method of fabricating a MOS transistor for a unit pixel of a CMOS image sensor, an active region may be defined by forming an isolation layer in a semiconductor substrate using an isolation process. For example, a shallow trench isolation (STI) process or a local oxidation of silicon (LOCOS) process may be used. When the active region is defined by the isolation layer, electrons may be trapped because a dangling bonding of silicon is broken at an interface between a silicon substrate and an oxide layer as the isolation layer. The unwanted trapping of electrons may lead to the occurrence of noise, which may degrade the characteristics of the image sensor.